The present invention relates to solid electrolyte voltaic oxygen gas sensors such as are operated at high temperatures to measure, for example, the oxygen content of automotive exhaust gases or boiler gases. Such sensors are formed of zirconium dioxide or other oxygen ion transferring composition and are normally stabilized by the addition of oxides of calcium, magnesium, yttrium or the like, as known in the art. Electrodes are formed on both the outside and the inside surfaces of the electrolyte body, generally thimble shaped, such as by applying to the surfaces porous layers of platinum. The outer electrode is exposed to the gas, the oxygen content of which is to be measured, such as automotive exhaust, while the inner electrode is exposed to a reference gas, generally air which contains about 21% oxygen. The air from within the tubular body thus serves as a reference gas establishing a fixed concentration of oxygen, hence a fixed reference voltage on the inner electrode. Any changes that occur in the voltage difference between the inner and outer electrodes therefore reflects a change in the composition of the gas, the oxygen content of which is measured, such as an exhaust gas.
When air is used as a reference, the sensor will give a very low voltage output when the exhaust gas has enough oxygen to exceed the chemical stoichiometric amount needed to react with all the oxidizable materials in the exhaust gas. When the amount of oxygen in the exhaust gas becomes less than the stoichiometric amount required, the voltage of the sensor will suddenly increase approaching 1 v or more as the net amount of oxygen becomes very small. This sudden change in voltage at the stoichiometric oxygen concentration is highly desirable for most applications of this type of sensor. The direction of the voltage change, however, as well as the use of air for the reference do have disadvantages.
One major disadvantage of the sensor voltage increasing when the oxygen concentration of the gas being measured decreases is that it precludes the use of a simple "fail-safe" operation with the sensor. Since a low voltage indicates excess oxygen, control circuits based on such an oxygen sensor automatically call for more fuel, or less air, to balance the combustion process, i.e. they call for a richer combustion mixture. If the oxygen sensor, for some reason, would become inoperative, its voltage output would ordinarily drop to zero or to a very low voltage. Such an output would automatically signal the control circuit to continue enriching the combustion mixture leading to an undesirable increase in air polluting compounds in the exhaust gas. An additional advantage exists when the oxygen sensor is used to help control automotive exhaust mixtures since richer fuel mixtures generally give better automobile driveability and there would be no incentive for the operator to replace a defective sensor.
Another disadvantage of the increase in output voltage of a sensor with a decrease in the oxygen content of the gas being measured is that such a sensor cannot directly replace or be replaced by a resistive-type sensor. A resistive-type oxygen sensor, such as a titanium dioxide oxygen sensor, increases resistance and therefore output voltage in the measuring circuit when the oxygen content increases. The direct interchangeability of voltaic-type oxygen sensor elements of zirconium dioxide with the resistive-type titanium dioxide sensor elements, while a property desired by automotive engineers, is not possible due to the reverse relation between oxygen concentration of the exhaust gas and voltage output evidenced by the two types of sensors.
The use of air as a reference gas in voltaic oxygen sensors has the disadvantage that any small leak or permeability of the sensor electrolyte body leads to a change in the reference side voltage so that it approaches that of the exhaust gas side. Also, if the reference side is deprived of replacement air, such as by blocking of a breathing hole for example, or is supplied with contaminated air, the reference side voltage will change and thus change the sensor output giving an inaccurate reading as to the oxygen content of the gas being measured.
An object of the present invention is to produce a voltaic oxygen sensor element that will overcome the aforementioned disadvantages without basic changes in the overall sensor design. In the present invention, a coating is applied to the inner surface of the solid electrolyte body, the coating being formed of a material which forms an oxide which is stable at operating temperatures of the sensor element, so as to protect the inner surface from contact with oxygen-containing gases within the tubular body, thus replacing the commonly used air reference with a very low, essentially zero, oxygen concentration reference. The reference side of the solid electrolyte body is thus covered with a material, stable at the operating temperatures of the sensor, that will react with, bind or remove from contact with the reference side of the electrolyte body, any oxygen present or appearing on the reference side.
With an effective zero oxygen concentration reference, the voltaic-type zirconium oxygen sensor would show a direct rather than a reverse voltage output relation to the oxygen concentration in the exhaust gas. When less than the stoichiometric amount of oxygen was present in the gas on both sides of the electrolyte body, the electrolyte would see very low concentrations of oxygen so this difference in voltage, the sensor output, would be small. When the oxygen content of the exhaust gas exceeded the stoichiometric amount the voltage would suddenly increase and would again approach 1 v or more as the oxygen content increased. The "zero" oxygen reference sensor would, in effect, operate as the mirror image of the present air reference sensor. Consequently, a low voltage output would indicate a low oxygen content in the exhaust gas and call for more air, or less fuel, in the combustion mixture. This would give a direct fail-safe feature to the sensor. In addition, with this relationship between voltage output and the oxygen content of the exhaust gas, these voltaic sensor elements would be interchangeable with resistive-type sensor elements.